The present invention relates generally to a laminated retardation layer, its fabrication process, and a liquid crystal display incorporating the same, and more particularly, a retardation layer for making the viewing angle of a liquid crystal display larger and improving color reproducibility, its fabrication process, and a liquid crystal display that incorporates the same.
Featuring slim size, low weight, low power consumption and flickerless, color liquid crystal displays (liquid crystal displays referred hereinafter to as LCDs for short) have increasingly expanded market, especially in the form of notebook computers. As part of PC-dedicated displays, there is now a growing demand for desktop monitors that are by far larger than notebook computers. Further, LCDs are finding applications for just only PCs but also TVs for which CRTs have so far been primarily used.
A problem with LCDs here is that their viewing angle is narrow for two possible reasons; when a VA (vertical alignment) mode LCD is viewed from an oblique direction, the isotropy of liquid crystal molecules goes out of order, causing light that, by definition, is to be linearly transmitted as such, to be converted to elliptically polarized light, and given crossed Nicols sheet polarizers located on both sides of a liquid cell, they appear to go out of crossed alignment upon viewing from an oblique direction. As a result, there are leakages of light out of pixels that are originally designed for black display, which may otherwise flip over contrast, rendering proper display impossible. To counteract such problems, a wide-viewing-angle, vertical alignment mode LCD has been figured out, which makes use of a retardation layer to prevent leakages of light out of black display pixels even at an increased viewing angle, as set forth in patent publications 1 and 2 as well as non-patent publication 1. For the chief purpose of removal of the problem arising from the fact that the isotropy of liquid crystal molecules goes out of order upon viewing from an oblique direction, a retardation layer called a negative C-plate (as will be described later in later) is used (see patent publication 3). For the primary purpose of removal of the problem resulting from the fact that crossed Nicols sheet polarizers appear to go out of crossed alignment upon viewing from an oblique direction, retardation layers called a positive A-plate and a negative C-plate (as will be describe later in detail) are used (see non-patent publication 2).
For the positive A-plate, it is proposed to use stretched polymer films having inverse chromatic dispersion, as set forth typically in patent publications 4, 5, 6 and 9 as well as non-patent publication 3. In particular, film products proposed in patent publication 9 and non-patent publication 3 are now on market in the trade name of Pure-Ace (Teijin Limited). Patent publication 10 discloses the lamination of two stretched films having different chromatic dispersions, and non-patent publication 6 shows that with such technology, a composite film having inverse chromatic dispersion is obtainable. Non-patent publication 1 teaches that if a stretched polymer film having such inverse chromatic dispersion is used as the positive A-plate for elimination of the problem stemming from the fact that upon viewing from an oblique direction, the crossed Nicols sheet polarizers appear to go out of crossed alignment, it is then possible to hold back color shifts upon viewing a VA mode color LCD from an oblique direction.
On the other hand, non-patent publication 4 proposes that the negative C-plate may comprise a polymerizable chiral nematic (cholesteric) liquid crystal with reflection wavelengths lying in the ultraviolet region, and non-patent publication 5 proposes that the negative C-plate may comprise a discotic liquid crystal of homeo-tropic orientation.
Patent Publication 1
JP(A)10-153802
Patent Publication 2
JP (A) 11-258605
Patent Publication 3
JP(A)10-312166
Patent Publication 4
JP(A)2000-137116
Patent Publication 5
JP(A)2002-14234
Patent Publication 6
JP(A)2002-48919
Patent Publication 7
JP(A)7-258638
Patent Publication 8
JP(A)10-508882
Patent Publication 9
WO00/26705
Patent Publication 10
JP(A)2-120804
Patent Publication 11
JP(A)2000-190385
Non-Patent Publication 1
IDW '02 pp. 525-527
Non-Patent Publication 2
SID 00 DIGEST pp, 1094-1097
Non-Patent Publication 3
IDW '00 pp. 407-410
Non-Patent Publication 4
IDW '00 pp. 413-416
Non-Patent Publication 5
SID 00 DIGEST pp. 1091-1093
Non-Patent Publication 6
SID 01 DIGEST pp. 566-569
Usually, when the negative C-plate and positive A-plate are located in an LCD to improve its viewing angle characteristics, they are laminated on sheet polarizers with optical axes at specific angles (absorption axes in the case of sheet polarizers and optical axes in the case of retardation layers). However, the tackifier used to this end is different in the index of refraction than the sheet polarizers and retardation layers, offering problems in that light is reflected at their interfaces, causing the contrast of images on display to become generally low and interference variations.
Especially when the negative C-plate and positive A-plate are located on both sides of a liquid crystal cell in accordance with non-patent publication 1, some more index interfaces add up, rendering the aforesaid low-contrast and interference variation problems even more noticeable.
According to non-patent publication 1, the positive A-plate having inverse chromatic dispersion may be used to prevent color shifts upon viewing from an oblique direction. However, there is still a lot of uncertainty about what type film is to be used as the negative C-plate for elimination of the problem resulting from the fact that the isotropy of liquid crystal molecules in the liquid crystal cell goes out of order upon viewing from an oblique direction; there is still a possibility that color shifts may remain uncorrected.